1. Field of the Invention
The present invention relates generally to the fields of developmental biology and molecular biology. More particularly, it concerns proteins involved in the regulation of cardiomyocyte cell growth and development.
2. Description of Related Art
The leading cause of morbidity and mortality in industrialized countries is heart disease, particularly heart disease that is associated with myocardial infarction. Myocardial infarction results in the loss of cardiomyocytes. Cardiomyocytes are post-mitotic cells and generally do not regenerate after birth. Furthermore, it has been discovered that they respond to mitotic signals by cell hypertrophy (Kodama et al., 1997; Pan et al., 1997) rather than by cell hyperplasia. The loss of cardiomyocytes leads to regional contractile dysfunction. In addition, the necrotized cardiomyocytes in the infarcted regions in the ventricular tissues are progressively replaced by fibroblasts to form scar tissue.
Recently, fetal cardiomyocytes transplanted in heart scar tissue limited scar expansion and prevented postinfarction heart failure (Leor et al., 1996). Although the transplantation of fetal cardiomyocytes is a proposed treatment of heart failure, it remains impractical in the clinical setting, in part because of the difficulty of obtaining fetal heart donor tissue. Thus, it is desirable to develop a cardiomyogenic cell line that could be used to facilitate the understanding of cardiomyocyte development and to facilitate the treatment of heart diseases, such as those associated with loss of cardiomyocytes.
Although it is known that the loss of post-mitotic cardiomyocytes results in increased morbidity and mortality, very little is known about the genes that are involved in heart development. It is known that transcription factors such as d-HAND, e-HAND (Srivastava et al., 1995), MEF-2C (Edmondson et al. 1994; Lin et al. 1997), Nkx2.5/Csx, GATA4, and TEF-1 play important roles in cardiac development (Harvey, 1996), but the lack of a model for cardiomyocyte differentiation has hindered the understanding of the interactions of these genes.
A recent report revealed that murine marrow stromal cells that are treated with 5-azacyidine, a cytosine analog capable of altering expression of certain genes that may regulate differentiation, results in a cell line that differentiates into cardiomyocytes in vitro (Makino et al., 1999). This cardiomyogenic cell line demonstrated several phenotypic characteristics that are specific to cardiomyocytes, e.g., adjoining cells via intercalated discs, forming myotubes, and beating spontaneously. In addition, the expression of cardiomyocyte specific genes, such as homeobox gene Nkx2.5, alpha-myosin heavy chain and atrial natriuretic factor, also are considered characteristic.
Although the proposed transplantation of fetal cardiomyocytes and cardiomyogenic cell lines are possible treatments, it is preferable to discover a treatment that eliminates any donor/species problems. Thus, identifying new regulators of cardiomyocyte growth and differentiation is an important goal in the search for therapeutics to treat myocardial tissue damage.